Multiple core processors include hierarchical power domains. Portions of the processor not needed to support a current level of activity are placed in a sleep state by powering them down, a technique referred to as power gating. The power domains are hierarchical in the respect that a sub-domain within a parent domain may be separately enabled or disabled while its parent domain is active. However, when the parent domain is power gated, the sub-domain is also power gated.
This ability to selectively power gate components within domains or sub-domains in an integrated circuit device is particularly important in handheld devices including, but not limited to, cell phones, personal digital assistants, portable entertainment systems, etc. In such devices, reducing the net power consumption of the device lengthens the amount of time between charges (or between replacements) of a battery power source. However, it is recognized that the selective ability to switch components on or off in an integrated circuit device is also important to traditional computer systems that are not dependent upon a battery source. For instance, laptops are often designed to dissipate the least amount of heat so that the user is comfortable handling the system. It may further be valuable to selectively power gate components to reduce the net power consumed on a traditional computer system. It is further recognized that the physical size of an integrated circuit device and/or computer system, and the amount of operating noise associated with the device/system, may also decrease as the number and size of heat sinks and fans is reduced.
The logic that controls and distributes the power gating signals is disposed in an always-on domain. An always-on domain is one that is not selectively power gated. This domain retains power at any time the system is powered on, even if some or most of the system components are in a steep state. The components disposed in the always on-domain are ones that are required to control the sleep states of the other domains. Affirmatively controlling the components that determine the power state of power gated domains avoids situations in which the logic signals driving the power gates could become indeterminate when a particular domain or sub-domain is powered down. This arrangement requires the always-on power domain to be physically distributed to the power gating logic within the domain or sub-domain. The distribution of the power and control for the power gating logic leads to increased leakage as the logic is in the always-on domain and cannot be shut off even if the domain and its sub-domains are power gated.
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